I can’t name the source. Incommunicable personal experience. Disregard it if you want. And it’s not a word for word quote. A different language was used and the concept of hope was understood with more depth than Thanos was referring to. It may have been more of a “I know that you will be remembered” or “the purpose of your destruction is to be remembered.”

But you’re right. I’m sorry, but I cannot source this for you, so feel free to disregard.